[TechAssist] Re: RCA CTC177BE tuning problem
- From: "Gonzalo Arrivas" <gna2000@xxxxxxx>
- To: techassist@xxxxxxxxxxxxx
- Date: Wed, 09 Jan 2002 20:19:37 +0000
Dave,
Here's a tuner troubleshooting you can follow:
TROUBLESHOOTING THE TUNER
IN THE CTC 175-187 CHASSIS
Step 1 Check the voltage at U7401-10 (5VDC)
This will verify B+ and establish a good ground.
Step 2 Monitor the voltage at Q7401-C Tuner Control
If the voltage is changing when you change channels this would
indicate U7401 is working.
Step 3 Check the voltage at U7501-4 Quad Op Amp
This is the 33 volt used for channel tuning.
Step 4 Monitor the voltage at Q7402-c Band Switch Transistor
You should have a negative DC voltage for band 1 (-11 to -14 VDC)
You should have a positive DC voltage for band 2 (+11 VDC)
This will switch positive or negative as you change bands.
Step 5 Monitor the voltage at U7401-9 Mixer/OSC
This is the band switch used at U7301-7. 3 VDC activates the VHF OSC at
pin 11. When UHF is selected, U7301-7 should go to 0 VDC. The UHF OSC
at
U7301-14 should be running. When OSC isn't running the voltage is about
10 to 11 VDC and when the OSC is running the voltage will measure about
4.5 to 5 VCD. Another good thing to know is that if U7301-1 is low
(less than 5 VDC) the chip is probably bad and the tuner alignment won't
respond because the RF AGC will remain high.
Step 6 The voltage at U7401-9 is also used for RF Band Switch
Q7403 and Q7404 collectors are connected together and the state of the
common
collectors determines the VHF/UHF RF switching. When the voltage at
U7401-9
is high (7 VDC), Q7403 turns on. Its collector is 0 VDC and the VHF RF
amp
Q7102 is turned on by lowering the source voltage to about 4.5 to 5 VDC.
When U7401-9 is low (0 VDC), Q7404 turns on. The collector is high (11
VDC).
Q7102-S will go high turning the UHF RF amp off. At the same time
Q7101-D (UHF RF amp) goes high and the UHF RF amp is turned on. An
overview
would be when a VHF band is selected, U7401-9 is 7 VDC. You should
have 3
VDC
at U7301-7. The VHF Mixer/OSC will be running and the voltage at
U7301-11
will
be about 4.5 to 5 VDC. Q7403 will be in saturation and the collector
is at
0 VDC. When U7401-9 is 0 VDC, U7301-7 is 0 VDC and the UHF Mixer/OSC is
selected. U7401-14 will now measure about 4.5 to 5 VDC. Q7404 will
turn
on,
collector is high and the UHF RF amp is selected.
Step 7 Q7401 off U7401-14 is the Tuning PLL Transistor
Its collector is used to control the 33 VDC for tuning. This voltage
is
used
at U7301-3 as a tuning reference for the QUAD OP AMP. The digital
information
stored in the eeprom comes into U7501 pins 5,10 and 12. This
information
is
converted to an analog value at pins 7,8 and 14 and used to tune the
varactor
diodes to band pass the selected channel frequency. You can use a DVM
to
measure
the voltage at U7501 pin 3. This is the tuning reference. Selecting
channel
2, you should have about 1.35 VDC on pin 3 and about 1 VDC at pins 7,8
and
14.
Selecting channel 13, the voltage at pin 3 is about 2.01 VDC. Pins 7,8
and
14
about 6 VDC. Now you should go to the varactor diodes and measure
approximately
the same voltage at their cathodes. (Keep in mind that these voltages
will
change
as you chage channels). When in the air mode, you may use the chart at
the
bottom
page 49 in the CTC 175/176/177 training manual for reference. When in
the
cable
mode, use the chart on page 51.
VARACTOR DIODES are designed such that their capacitance is dependent
on
the voltage
across the diode. As the tuning voltage varies across the diode, a
varactor acts
like a variable capacitor causing the tuned circuit to vary its
frequency.
Different
channels are tuned in this manner. (Note: Varactor diodes work with
reverse bias).
Thanks to Bob Rose for providing the above information...
>From: "Dave Huston" <DaveHuston@xxxxxxxxxxxx>
>Reply-To: techassist@xxxxxxxxxxxxx
>To: <techassist@xxxxxxxxxxxxx>
>Subject: [TechAssist] RCA CTC177BE tuning problem
>Date: Tue, 9 Jan 1996 07:47:11 -0500
>
>
>This one tunes channels 2-13 fine. UHF channels won't change. I get the
>same
>channel no matter what the OSD says. Did standard RCA tuner resolder to no
>avail. Has anyone seen this before?
>Dave
>
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G.N.Arrivas, CET, Pittsburg. CA
Tel/Fax: 925-432-2057
Email: gna2000@xxxxxxx
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